Dipeptidylpeptidase-IV (DPP-IV) is a serine protease which cleaves N-terminal dipeptides from a peptide chain containing, in general, a proline residue in the penultimate position. DPP-IV is widely expressed in mammalian tissue as a type II integral membrane protein. The protease is expressed on the surface of differentiated epithelial cells of the intestine, liver, kidney proximal tubules, prostate, corpus luteum, and on leukocyte subsets such as lymphocytes and macrophages. A soluble form of the enzyme is found in serum that has structure and function identical to the membrane-bound form of the enzyme but lacks the hydrophobic transmembrane domain.
DPP-IV has many physiologically relevant substrates including chemokines (e.g. eotaxin and macrophage-derived chemokine), neuropeptides (e.g. neuropeptide Y and substance P), vasoactive peptides, and incretins (e.g. GLP-1 and GIP). GLP-1 (glucagon-like peptide-1) is a hormone produced in the L cells of the distal small intestine in response to ingested nutrients. GLP-1 receptor binding on various tissues stimulates insulin gene expression, biosynthesis and glucose-dependent insulin secretion, inhibits glucagon secretion, promotes satiety, slows gastric emptying and promotes growth of pancreatic beta cells.
Although the biological role of DPP-IV in mammalian systems has not been completely established, it is believed to play an important role in neuropeptide metabolism, T-cell activation, attachment of cancer cells to the endothelium and the entry of HIV into lymphoid cells. It has also been discovered that DPP-IV is responsible for inactivating glucagon-like peptide-1 (GLP-1). Since GLP-1 is a major stimulator of pancreatic insulin secretion and has direct beneficial effects on glucose disposal, DPP-IV inhibition appears to represent an attractive approach for treating, for example, non-insulin-dependent diabetes mellitus (NIDDM).
DPP-IV has also been shown to play a part in the immune response. Expressed by T-CD4+ lymphocytes, where it is synonymous with the antigen CD26, DPP-IV plays an important part in the mechanism of transplant rejection (Transplantation 1997, 63 (10), 1495-500). By allowing more selective suppression of the immune response, inhibition of DPP-IV accordingly represents an extremely promising approach in the prevention of transplant rejection in transplant patients.
Inhibitors of DPP-IV are described inter alia in WO-A-2003/068748, WO-A-20031068757, US-A-2006/0014764 and U.S. Pat. No. 2,005,10272765.